1. Field of the Invention
The present invention relates to a method for implanting the sidewalls of isolation trenches and, more particularly, to a method for implanting trench sidewalls which utilizes a series of ion implants and a subsequent narrowing of the window through which the trench is formed to provide a sufficient level of dopant concentration in the trench sidewalls.
2. Description of the Prior Art
Advanced large scale integrated circuit design requires a method of providing isolation between various components located on the same substrate which does not demand a large surface area of the substrate. Trench isolation has evolved as the preferred method of isolation, where narrow trenches, filled with a dielectric, can be placed between components and provide excellent isolation between components which are closely spaced. A problem with this method, however, relates to the dopant segregation from the substrate during a brief oxidation process along the trench, where this oxidation is required to keep a clean interface. For example, in a boron doped p-type substrate which includes an isolation trench filled with silicon dioxide, the boron will move from the silicon substrate into the silicon dioxide. Moreover, an inversion may occur along the portion of the silicon substrate which borders the silicon dioxide-filled trench, due to the positive fixed charge. Therefore, in the case where the trench is being utilized to separate two closely spaced n-type regions, the presence of the n-type inversion layer between the regions serves as a conduction path, thus degrading the isolation between the two regions.
One method of overcoming this problem is to separate the components to be isolated by an amount sufficient to disrupt the conduction path around the trench sidewalls. However, when a large number of components need to be separated from one another on a single substrate, the amount of silicon area required for this method becomes extremely costly. In an alternative method, a "channel stop" is placed at the bottom of the trench to break the conduction path between the two regions to be isolated. As disclosed in U.S. Pat. No. 4,211,582 issued to C. T. Horng et al. on July 8, 1980, a p+ implant is formed at the bottom of the trench to prevent an inversion layer from forming at the bottom of the trench along the interface with the substrate. In particular, by enforcing the presence of p-type donors, the boron concentration in the silicon substrate will be sufficient to prevent the interface from inverting, even after the boron segregation during the subsequent oxidation. Therefore, the likelihood of an n-type inversion layer being fromed at the bottom of the trench is significantly reduced.
Although this method is sufficient for eliminating the invention layer at the bottom of the trench, the boron segregation which takes place along the sidewalls of the trench may result in forming an inversion layer around the sidewalls of the isolation trench for the same reasons as stated above. Therefore, a conduction path still exists between the two components via the inverted sidewalls. A problem remaining in the prior art, therefore, is to provide a method for doping the sidewalls of an isolation trench which prevents any sidewall inversion and provides complete isolation.